Input: mt6779-keypad - support double keys matrix

Input: mt6779-keypad - support double keys matrix

MediaTek keypad has 2 modes of detecting key events:
- single key: each (row, column) can detect one key
- double key: each (row, column) is a group of 2 keys

Double key support exists to minimize cost, since it reduces the number
of pins required for physical keys.

Double key is configured by setting BIT(0) of the KP_SEL register.

Enable double key matrix support based on the mediatek,keys-per-group
device tree property.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220720-mt8183-keypad-v2-5-6d42c357cb76@baylibre.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>